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Materials 2 > Metal Forming > Flashcards

Metal Forming Flashcards

1
Q

What is the primary mechanism of change in metal forming?

A

Plastic deformation due to mainly compressive stresses.

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2
Q

What material properties are desirable when metal forming?

A

Low yield strength, high ductility

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3
Q

What are the two main categories of metal forming? What are the main processes within each category?

A 
Bulk deformation :
     Rolling
     Forging
     Extrusion
     Wire and Bar Drawing
Sheet metalworking
     Bending
     Deep drawing
     Shearing
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4
Q

What characterizes bulk versus sheet metalworking?

A

Surface area to volume ratio

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5
Q

What is the stress strain relationship in the plastic region?

A

sigma = K epsilon^n

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6
Q

What is the flow stress? The average flow stress?

A

Yf is the stress at a single moment in time, Yavg = Yf/1+n

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7
Q

Describe cold work. Advantages? Disadvantages?

A

Room temp or slightly above
Minimal machining required (net shape process)
Good accuracy and tolerance and surface finish
Strain hardening
Grain flow causes directional properties
Requires higher force
Starting surfaces must be clean
Ductility limits amount of work that can be done, sometimes requires annealing to complete.

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8
Q

Describe warm work. Advantages?

A

Between 0.3 and 0.5 Tm.
Lower forces required
More intricate geometries possible
Little to no annealing needed

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9
Q

Describe hot work. Advantages? Disadvantages?

A 
Above 0.5Tm
Substantial plastic deformation possible (because K is low, and n = 0, high ductility)
Lowest force required
Metals are unlikely to fracture
Properties are isotropic
No strain hardening.
Lower accuracy and surface finish
High energy required
Short tool life
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10
Q

Describe strain rate sensitivity

A

Hot worked metals should be perfectly plastic, and flow at a given stress, but instead experience strain based on rate of deformation
epsilon = v/h, where v is speed of ram and h is instantaneous height of material.
When hot worked, metals have a flow stress that depends on strain rate. When strain rate goes up, so does resistance to deformation.

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11
Q

At what temperatures is friction most severe.

A

High ones.

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12
Q

Why is friction bad?

A

Reduces metal flow
Increases required force
Wears out tools

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13
Q

What does lubrication do?

A

Reduces friction, heat in tooling, and improves surface finish.

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14
Q

In rolling, what are the main functions of the rolls?

A

To pull the work into the gap and to compress it.

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15
Q

What is flat rolling? Shape rolling?

A

Flat - reduce thickness of a rectangular cross section

Shape - change cross-section (ex. bar into I-beam)

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16
Q

In rolling, what is the draft? The reduction?

A 
d = to - tf
r = d/to
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17
Q

Describe the different mill configurations

A

2 high : two simple rollers
3 high : work passes through bottom, then reverses direction and goes through top (or vice versa)
4 high : top and bottom rollers simply support the rollers doing the work
Cluster : multiple rolls backing the main ones
Tandem : sequence of various two-high mills
Thread rolling : always cold work

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18
Q

Define impact forging vs press forging

A

Impact - sudden force (usually gravity, sometimes accelerated by pressurized air or steam)
Press, gradual force (hydraulic, mechanical, screw)

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19
Q

What are the three main types of forging. Describe them

A

Open die : two flat dies, work is allowed to flow laterally
Impression die : die with cavity, flash is created
Flashless : Die completely constrains work, no flash created

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20
Q

What is barreling?

A

When friction prevents work from expanding at the die, and therefore the work ‘barrels’ out in the center. Is most sever in hot working because work cools on the dies.

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21
Q

Why is the flash important in impression-die forging?

A

Flash has high friction, and constrains remainder of metal into desired shape.

22
Q

Is impression die forging done in one step or several?

A

Several, to go from large bulk work to final shape.

23
Q

What are the advantages and disadvantages of impression die forging?

A 
High production rates
Little waste of material
Large strength
Favorable grain orientation
Bad tolerance
Machining required afterwards
24
Q

What is upsetting?

A

Forging a head onto a bolt or screw

25
Q

Define swagging. Define radial forging.

A

Yo bro.
Or,
Swagging - rotating dies to taper work
Radial - rotating work to taper it

26
Q

What is trimming?

A

Cutting off the flash

27
Q

What direct extrusion? Indirect Extrusion?

A

Direct - Ram pushes work through die. Final product is called extrudate. Excess is called butt.
Indirect - Die is forced into the work. Not as effective because of lower rigidity of hollow ram, and difficulty in supporting extrudate.

28
Q

What is the reduction ratio for extrusion?

A

rx = Ao/Af

29
Q

What are the parts of an extrusion die?

A

The container, die, and die angle.

30
Q

Why is die angle important?

A

Affects friction and therefore ram force.
Too low, and friction is too high.
Too high, and turbulence in metal flow increases (which increases ram force)

31
Q

What is wire/bar drawing?

A

Extrusion, but pulling. Bar starts with larger stock, wire with smaller. Usually cold work only. Both tensile and compressive stresses involved.
Bar is a single die operation, wire is often multiple.
Wire often includes annealing between dies.

32
Q

What is the reduction ratio for drawing?

A

r = (Ao-Af)/Ao

33
Q

What are the parts of a draw die?

A 
Entry (funnels lubricant into die)
Approach (cone shaped area where drawing occurs)
Bearing surface (determines final cross section)
Back Relief (exit zone, usually 30 degrees)
34
Q

How is the work prepared for drawing?

A

It is annealed, cleaned, and pointed.

35
Q

What are the advantages of sheet metal parts?

A

High strength
Good accuracy, surface finish
Low cost
Good mass production

36
Q

What are the four parts of the fracture surface of a cut part?

A

Rollover
Burnish
Fracture zone
Burr

37
Q

What is clearance?

A 
The space between the punch and die
C=at
C is usually 4-8% of t
If too small, more force is required and double burnishing occurs
If too large, creates large burr
38
Q

Describe shearing

A

Splitting a material with an inclined blade.

Shear angle decreases required force

39
Q

Describe blanking and punching

A

Blanking - Cutting out the final shape

Punching - Cutting out of the final shape

40
Q

How are the diameters of die, punch, and blank related in blanking?

A

Blank has Db
Punch has Dp=Db-2c
Die has Dd=Db

41
Q

How are the diameters of die, punch, and hole related in punching?

A

Hole has Dh
Punch has Dp=Dh
Die has Dd=Dh+2c

42
Q

What is the force required for shearing?

A

F = S t L

43
Q

What are the two types of sheet metal bending?

A

V bending

Edge bending

44
Q

What contributes to stretching of material during bending?

A

Small bend radius compared to thickness.

45
Q

Define springback

A

When bend angle decreases and bend radius increases after load is removed : caused by elastic deformation in part.

46
Q

What is deep drawing? What clearance is used?

A

Pushing sheet metal into hollow or complex geometries (ex cup or box shaped)
c = 1.1 t

47
Q

What is the drawing ratio? Reduction? Thickness to diameter ratio?

A

DR = Db/Dp <= 2
r = (Db - Dp) / Db < 0.5
t/Db >= 1%

48
Q

When does wrinkling occur in drawing?

A

When t/Db is too small

49
Q

How is blank size determined for drawing?

A

Db is solved by assuming constant volume and negligible thinning.

50
Q

What’s the difference between a gap press and a brake press.

A

Size. Brake presses are ginormous.

A CNC press is a gap press

51
Q

When is a hydraulic press most useful? How about mechanical?

A

Hydraulic - slow, long stroke, good for deep drawing.

Mechanical - fast stroke with high force at bottom, good for blanking and punching.

52
Q

What is the state of stress for rolling? Forging? Extrusion? Swagging? Deep drawing? Wire drawing?

A

Rolling - Biaxial compression
Forging - Triaxial compression
Extrusion - Triaxial compression
Swagging - Biaxial compression
Deep drawing - Biaxial tension and compression in flange, uniaxial compression in wall of cup
Wire drawing - Biaxial compression, tension

Materials 2 (12 decks)

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